Alastair McIndoe, Straits Times 29 Apr 08;
MANILA - MOSQUITOES that spread malaria have started bugging people more and more in places such as Papua New Guinea's cooler highlands, where they were once rare.
Now scientists are wondering if the growing health menace may be another sign of global warming.
'Malaria-carrying mosquitoes are now being found in areas where there was no malaria before,' said Dr Shigeru Omi, the World Health Organisation's (WHO) Manila-based regional director for the Western Pacific.
In both the northern and southern hemispheres, disease-carrying mosquitoes are spreading from the tropics to cooler climates.
Papua New Guinea's cooler highlands, for instance, where around 40 per cent of the population lives, were mostly free of malaria until the 1998 El Nino weather phenomenon brought unseasonably high temperatures.
Malaria took hold that year and epidemics are now a seasonal occurrence in this western Pacific island where temperatures have risen by 1 deg C over the past two decades.
To what extent global warming is to blame is being intensely debated by international health experts and governments in countries at risk.
'While a number of factors are at play, there seems little doubt that rising temperatures and unseasonably high rainfall have a role,' said the Japanese-born Dr Omi.
Those factors include the impact of human activity on climate change - through deforestation, for example - as well as non-climatic events, like the increased flow of people across national borders.
The WHO estimates that at least 150,000 people die each year from mosquito-borne diseases, diarrhoea, malnutrition and extreme weather events which can be traced to climate change. Half of those deaths are in Asia.
The health organisation marked World Malaria Day last Friday with a global campaign on the theme: A Disease Without Borders.
'It is time to recognise that even the safest national borders cannot protect us from global threats such as malaria,' said Dr Ava Marie Coll-Seck, executive director of Roll Back Malaria, a global advocacy group.
While evidence is mounting of a connection between malaria and climate change, establishing a link with an increase in dengue fever in parts of the region is proving harder.
'If you look at Singapore, the number of dengue cases has gone up as the temperature rises, but it is still too early to conclude that this is due to climate change,' said Dr Omi, who nevertheless believes that global warming has been a contributing factor.
Annual temperatures in Singapore rose by 1.5 deg C between 1978 and 1998. Over that period, the number of mosquito-borne dengue cases increased more than tenfold, from 384 to 5,258, according to the WHO.
This year's dengue outbreak is shaping up to be Singapore's worst. Between Jan 1 and April 5, 1,305 people were struck by the disease, a 60 per cent increase over the same period last year.
Despite having the toughest anti-dengue measures in the region, the Singapore authorities are still searching for the cause of the surge.
Health officials in the Philippines, where there has also been a sharp rise in dengue cases, believe that warmer weather may be shortening the breeding cycle of mosquitoes.
There were 5,859 dengue cases in the first nine weeks of this year, a 17 per cent rise on the same period last year, the National Epidemiology Centre said.
Philippine Health Secretary Francisco Duque, quoted in the local media recently, noted that an increase in dengue cases - as well as other diseases - had occurred over a period of rising temperatures.
But he cautioned against a hasty cause-effect conclusion, saying: 'There is a relationship between climate change and human health, but this connection is very complex. We still need more research on this.'
In 1998, the same year that malaria broke out in Papua New Guinea's highlands, the Philippines recorded 36,000 dengue cases, its highest.
Preventive measures were stepped up. 'Barangays' - the rural and urban community units in which Filipinos live - are carrying out dengue awareness campaigns, even in impoverished areas. And dengue traps, such as tyres used to weigh down shanty roofs, are now much less prevalent.
Dr John Ehrenberg, the WHO's regional adviser for malaria and other vector-borne parasitic diseases, said that the characteristics of dengue epidemics are changing.
'In some places, you find the disease affecting mainly males, and one of the factors to consider is occupational,' he said.
'That's not difficult to understand because of the breeding sites, where men work and sleep, in Asia's construction boom.
'But the odd thing is that we are seeing the disease mainly affecting males across the age groups - and not just adults.'
The cycle of dengue outbreaks may also be changing, with the disease shifting from being 'extremely seasonal to a situation where you pretty much have a continuous presence', he said.
The WHO estimates that there may be 50 million cases of dengue around the world every year, half of which require hospitalisation. About 12,500 cases are fatal.
There has been a sizeable reduction of dengue cases in Asia as a whole: 760,600 in 1990 compared to 343,062 in 2005, though the fall has slowed noticeably since 2000.
Dr Omi worries that the declining trend could be reversed by climate change.
He warned: 'The exceptionally high number of dengue cases presently being seen in Asia may be giving us a glimpse of what could lie ahead.'
